In-situ Stress Perturbation due to Temperature around Borehole during Carbon Injection

Downhole temperature variation of CO2 plays an important role in underground CO2 sequestration. Along the wellbore, the temperature difference between surrounding country rock and CO2 can cause the stress perturbation to the pre-historic in-situ stress, restricting the commercial injection rate and the depth of the targeted reservoir. The aim of the present study is to develop a simplified heat flow model along the wellbore, with the intention to use the generated temperature differential to study the thermal stress perturbation downhole. This study found that stress perturbation to the in-situ pre-historic stresses due to the temperature differential is dictated by circumferential heat transfer coefficient around the borehole, injection velocity and borehole radius. It is found that caprock is more prone to failure than the storage formation. The caprock, which has Young’s modulus value twice than the sandstone, reduce the breakdown pressure by six times when subjected to the same temperature differential. In-situ stress anisotropy is also found to be critical to the lowering of the breakdown pressure when subjected to the same temperature differential. Under the same temperature disturbance, an increase in in-situ stress anisotropy lead to a lowering of the fracture initiation pressure. Both of these structural aspects put a preliminary upper limit to the maximum allowable downhole temperature variation, directly restricting the upper limit of the subsurface sequestration rate.